Nocturnal singing by diurnal birds in Afrotropical highlands

Abstract Most bird species are active by day. However, even typically diurnal species have been observed to sing by night, although the functions and mechanisms of this behaviour are not well understood. In our study, we focused on nocturnal singing by diurnal birds in Afrotropical highlands (Bamenda Highlands, Cameroon). We recorded 48-h soundscapes in 18 random recording sites using autonomous sound recorders to examine the occurrence and intensity of nocturnal singing among diurnal birds. We applied three definitions of night—the time period between: (1) astronomical, (2) nautical and (3) civil dusk and dawn, which may include or exclude beginning of dawn chorus from the night-time period. We recorded 53 bird species, representing seven orders and 27 families. Of 50 typically diurnal species, only three sang by astronomical night, producing in total 10 songs. Applying the nautical night definition did not change the meaning of the results—the same three species produced 12 vocalisations. By civil night, 21 species were recorded singing. From the analysed 2128 vocalisations produced by civil night, 2109 were produced during 25 min period between nautical and civil dawn (22–47 min before sunrise), what suggests that civil night covers the beginning of dawn chorus and should not be considered as a nighttime. Our study reveals that in Afrotropical highlands, unlike in temperate regions, diurnal birds stay silent at night. Using astronomical and nautical definition of night gives similar results. The intensive singing recorded between nautical and civil dawn (included into nautical night), we interpreted as the beginning of dawn chorus rather than true night singing. We suggest that precise definition of nighttime is required to compare the results of different studies and to better understand the phenomenon of night singing by diurnal birds.